1. Field of Invention
The present invention relates to a field of USB communication, and more particularly to a circuit for generating a USB peripheral clock and a method therefor.
2. Description of Related Arts
USB is the abbreviation for Universal Serial BUS. Due to the advantages such as high communication speed, simple interface and convenient application thereof, recently the USB has become one of the standard interfaces essential to electronic equipments such as PC, MP4, mobile phone, PDA (Personal Digital Assistant), digital camera, printer and scanner. The USB is widely applied in fields of information communication, data transmission and etc.
The conventional communication systems all require a clock source with relative accuracy. The clock source is then processed with logics of frequency dividing, frequency doubling and etc. in the communication system, so as to generate a master clock required thereof for operating. The master clock collects and analyzes the data stream transmitted thereof, so as to accomplish the object of data communication. The USB communication system is no exception, and in a high-speed USB communication (with communication speed equal to or greater than 480 MHz), the system requires that the clock for data transmitting has a high accuracy (±0.5%). Therefore, technical solution adopted by the USB communication system is to connect a crystal oscillator on an external of the main body of the USB, i.e., the crystal oscillator generates an accurate clock on an external thereof, 12 MHz for instance, so as to be inputted to the internal of the chip in the main body of the USB, and is processed with frequency doubling in the internal of the chip by logic modules such as PLL (phase-locked loops), in such a manner that a high-speed clock of 480 MHz required by the system for operating is finally generated.
Due to the crystal oscillator thereof, the chip of the main structure of the USB requires at least two pins spared thereof for the crystal oscillator. Thus, in the electronics relatively having a small number of pins, the USB communication system can not be utilized for communication. E.g., the conventional electronic products, SIM card for instance, generally have 4˜7 pins, due to the requirements of other system function, the electronics are not capable of sparing two additional pins for the crystal oscillator, and thus the main structure of the USB can not be utilized for communication.
Furthermore, with the development of the production process and the improvement of the design technique, the volume of the electronics tends to be smaller and smaller, and the number of the pins thereof tends to be smaller and smaller as well. However, compared with thereof the SOC (System on Chip), the volume of the crystal oscillator component are quite large, which will restrict the development of high integration and miniaturization of the electronics. Therefore, the crystal oscillator turns into a key factor which restricts the application and development of the chips of the USB main structure.
Of course, the clock can also be generated inside the chip of the USB main structure via an RC/LC oscillator, and is supplied for the system. However, due to influence of the process variation of the RC/LC oscillator or other factors, the clock finally generated by the crystal oscillator in the chip often has a deviation of ±20% with the design target, which is not capable of meeting the demand of the system for the accuracy of transmission.
The high-speed USB requires a higher accuracy of the peripheral clock than the full speed equipments (with a communication speed of 12 MHz) and low speed equipments (with a communication speed of 1.5 MHz), and furthermore the data code rate of peripherals of the high-speed USB is 480 Mb, which is much higher than peripherals of the full-speed or low-speed USB peripheral, so the clock generating circuit for the full-speed or low-speed USB peripherals is not suitable for the high-speed USB peripherals.
Therefore, it is necessary to provide an improved circuit for generating USB peripheral clock and a method therefor to overcome the shortcomings mentioned above.